1. Field of Endeavor
This invention relates to methods and apparatus for cleaning debris in confined areas including, for example, heat exchangers having vertically arranged tube arrays and, more particularly to methods and apparatus for removing sludge deposits from the tube sheets of steam generators using low-pressure, high-flow coherent fluid jets.
2. Description of the Conventional Art
In nuclear power plants, steam generators serve as large heat-exchangers for generating steam which is used for driving turbines. A typical steam generator has a vertically oriented outer shell containing a plurality of inverted U-shaped heat-exchanger tubes disposed therein to collectively form a tube bundle. The U-shaped tubes are commonly arranged in a triangular-pitch or square-pitch tube array to form interstitial gaps, or “intertube lanes,” that are typically from about 2.5 mm to 10 mm (about 0.1 to 0.4 in.) wide. In most steam generator designs, a centrally located, untubed region extending longitudinally along the central vertical axis of the steam generator is defined by the elongated portions of the innermost U-shaped tubes. This untubed region is typically about 10 cm (4 in.) wide and may be referred to as the “no-tube” lane.
A plurality of horizontally oriented upper annular tube support plates are provided at periodic intervals for arranging and supporting the U-shaped tubes. Each tube support plate typically contains a triangular- or square-pitch array of holes or openings therein for accommodating the elongated portions of the U-shaped tubes. The height of the U-shaped tubes may exceed 9.75 m (32 ft), and a conventional steam generator will typically include six or more tube support plates, with each tube support plate being horizontally disposed along the tube path with adjacent tube support plates typically having a vertical separation of 0.9 to 1.5 m (3 to 5 foot) intervals.
A tube sheet spaced below the lowermost tube support plate separates a lower primary side from an upper secondary side of the steam generator. A dividing plate cooperates with the lower face of the tube sheet to divide the primary side into an entrance plenum for accepting hot primary coolant from the nuclear core and an exit plenum for recycling lower temperature primary coolant to the reactor for reheating. The entrance and exit plenums are connected through the tube sheet by the U-shaped tubes.
Primary fluid that is heated by circulation through the core of the nuclear reactor enters the steam generator through the entrance plenum. The primary fluid is fed into the U-shaped tubes, which carry the primary fluid through the secondary side of the steam generator. A secondary fluid, generally water, is concurrently introduced into the secondary side of the steam generator and circulated through the interstitial gaps between the U-shaped tubes. Although isolated from the primary side fluid in the U-shaped tubes, the secondary fluid comes into fluid communication with the outer surface of the U-shaped tubes thereby transferring heat from the primary fluid to the secondary fluid. This heat transfer, in turn, converts a portion of the secondary fluid into steam that is then removed from the top of the steam generator in a continuous steam cycle. The steam is subsequently circulated through standard electrical generating equipment. The cooled primary side fluid exits the steam generator through the exit plenum, where it is returned to the nuclear reactor for reheating.
Under normal operation of a nuclear power plant, impurities such as iron and copper are transported to the steam generators via the secondary side feed water system. These impurities accumulate as scales on the outer diameter of steam generator tubing, as well as sludge, which settles on the upper surfaces of the tube support plates and on the tube sheet. These sludge and scale accumulations can lead to many unwanted side-effects including accelerated degradation of steam generator tubing and other internal components, and decreased heat transfer efficiency. As a result, it is desirable to periodically remove these sludge and scale accumulations in order to maintain steam generator cleanliness, integrity and performance.
The most commonly used method for removing the sludge collected on the tube sheet of steam generators is referred to as sludge lancing. Sludge lancing methods use high-pressure, for example 5.2-27.6 MPa (750-4,000 psi), water jets to dislodge the sludge. These water jets work in conjunction with corresponding suction and filtration equipment for removing and disposing of the sludge dislodged by the high-pressure water jets. In practice, these high-pressure water jets are directed into the 2.5 to 10 mm (0.1 to 0.4 in.) intertube lanes to dislodge and flush sludge that settles in the interstitial gaps formed between the tubes. The sludge-laden water is subsequently collected by suction equipment that may, in turn, be operatively connected to a filtration/recirculation system that may be used to separate the sludge from the sludge-water mixture for disposal.
Two principal types of lancing devices are used to clean steam generators in conventional cleaning operations. The first, and probably more common, type of lancing device comprises a high-pressure lance that is installed through access ports provided in the steam generator shell opposite both ends of the no-tube lane. This high-pressure lance is then used to dislodge sludge from within the tube bundle and flush sludge to the steam generator periphery where it is then collected and removed from the steam generator using suction equipment. As discussed in Hickman et al.'s U.S. Pat. No. 4,079,701, the efficiency of sludge collection at the steam generator periphery can be enhanced by establishing a circumferential flow around the tube bundle that will tend to direct sludge toward the suction equipment once it is flushed from the tube array boundary and reaches the steam generator periphery.
The second type of lancing device, sometimes referred to as an “outside-in” device, comprises a high-pressure lance that is installed through an access port in the annulus between the tube bundle and the steam generator shell. This lance is used to dislodge and flush sludge from the steam generator periphery toward the no-tube lane, or toward another region of the steam generator annulus, where the dislodged sludge may be collected and removed by suction equipment.
To some extent, both types of sludge lance devices described above are capable of removing soft, highly mobile sludge accumulations, which collect on the tube sheet in steam generators. However, the sludge removal efficiency of these devices is typically reduced by lateral scattering of the dislodged sludge. In particular, the high-pressure water jets used to dislodge sludge characteristically result in some lateral scattering of the dislodged sludge into areas of the tube array that have already been cleaned, rather than effectively flushing the sludge toward suction equipment intake. As a result, multiple passes and long application times are typically required to achieve satisfactory cleaning levels, even when the majority of the sludge present on the tube sheet is soft and highly mobile, i.e., is not highly adherent and/or consolidated.
As discussed in Lahoda et al.'s U.S. Pat. No. 4,676,201, this lateral scattering effect may be reduced when the height of the sludge pile on the tube sheet is about one inch or higher because sludge present in adjacent intertube lanes limits the spread of sludge and water from the intertube lane being processed. As a result, sludge lancing works well for reducing the height of large sludge piles (10 to 15 cm) (four to six inches deep, or more) to smaller sludge piles (2.5 cm deep, or less) (1 in. deep, or less). However, complete removal of these smaller sludge piles by sludge lancing is difficult due to a greater tendency for the high-pressure water jets to scatter the dislodged sludge into previously cleaned areas.
Because most nuclear power plants now operate with better water chemistry control, fewer impurities are transported to the steam generators during plant operation. However, even with good water chemistry control, small piles of sludge can accumulate on the tube sheet in the steam generators. If an All Volatile Treatment (AVT) chemistry is employed, the majority of the sludge that accumulates on the tube sheet within the steam generator will typically comprise soft, silt-like particulates. However, over time this soft, highly mobile sludge can harden/consolidate and form more tenacious deposits, i.e., hard sludge, often referred to as tube sheet “collars.”
High-pressure lancing techniques, however, have proven to be somewhat less effective for removing these more tenacious deposits. Indeed, chemical cleaning techniques and/or more aggressive mechanical cleaning techniques are typically required to remove the majority of these more tenacious deposits. As a result, many utilities are interested in removing these smaller piles, for example, deposits having a depth of about 2.5 cm or less (about 1 inch or less) of soft sludge before they consolidate, and would prefer to use a method or apparatus that is more efficient for removing small piles of soft, highly mobile sludge than available high-pressure water lancing techniques.
As discussed in Lahoda et al.'s U.S. Pat. No. 4,676,201 and Muller et al.'s U.S. Pat. No. 4,715,324, attempts to increase the efficiency of high-pressure sludge lancing techniques have led to modifications of several lancing devices to include both high-pressure water jet(s) for dislodging sludge and “barrier” jet(s) for preventing redeposition of scattered sludge in areas that have already been cleaned. However, there are several additional disadvantages associated with these modified designs. Specifically, the high-pressure water jet and the barrier jet in the apparatus described in the Lahoda et al.'s U.S. Pat. No. 4,676,201, the contents of which are hereby incorporated, in its entirety, by reference, are typically separated by a gap of at least two columns of tubes. This gap allows any sludge scattered by the high-pressure water jet to collect between the two jets, resulting in subsequent scattering by the barrier jet.
In the method described in the Muller et al.'s U.S. Pat. No. 4,715,324, the contents of which are hereby incorporated, in its entirety, by reference, the high-pressure water jet and low-pressure water jet are operated in an alternating manner, rather than simultaneously. As a result, little, if any, reduction in lateral scattering or increase in sludge removal efficiency is achieved by this method. Similarly, cleaning operations using this technique do not tend to result in little, if any, reduction in the number of passes or required application time would be expected. The shortcomings associated with the modified lancing devices described in both the Lahoda et al.'s U.S. Pat. No. 4,676,201 and Muller et al.'s U.S. Pat. No. 4,715,324, is reflected in the failure of devices according to these disclosures to achieve wide use within the industry and the continued widespread reliance on previous generation lancing devices.